949
FXUS62 KRAH 191748
AFDRAH
Area Forecast Discussion
National Weather Service Raleigh NC
140 PM EDT Mon Mar 19 2018
.SYNOPSIS...
Weak high pressure over the area will migrate offshore, allowing a
warm front to lift north into the area by evening where it will
stall tonight. There will be a series of low pressure systems
developing along the front as it pushes slowly south of the
Carolinas through early Wednesday.
&&
.NEAR TERM /THROUGH TUESDAY/...
As of 1120 PM Monday...
Cloudiness will continue to slowly thickine and lower in weak
isentropic upglide. Rain has been very light, with radar echoes
mostly not reaching the ground, hence will lower PoPs through the
evening when the lift will intensify ahead of the surface low moving
across the southern tier. Temps are close to forecast and no changes
of significance are needed.
Previous discussion: Surface high pressure will continue to migrate
east and offshore this morning. A warm front south of the area will
lift north into the area late this afternoon and evening, in advance
of an area of low pressure tracking east and into the Tn Valley.
Today: Initial surge of weak to modest low/mid-level warm moist air
advection, fueled by weak shortwave impulses embedded in the
westerly flow aloft, well downstream of the compact upper
low/vigorous shortwave trough moving east through the central Plains
and Mid MS Valley, will support patchy light rain across the area
today. There could potentially be a mid day/afternoon lull in precip
before the arrival of the stronger moisture advection and lift this
evening and overnight, owing to the approach of the aforementioned
shortwave trough and associated sfc low into the Tn Valley.
Forecast highs will be highly dependent on just how much precip we
see and if it will linger into the afternoon, especially across the
far northern Piedmont where sufficient precip falling into the
residual low-level dry air will help to lock in in-situ wedge
conditions. Highs ranging from lower to mid 50s north to lower 60s
south.
Tonight: As the area of low pressure tracks east into the Tn
Valley/southern Appalachians, sfc warm front will lift north into
the area. Strong southerly/southwesterly warm moist air advection
rooted in a 40-50 kt LLJ overspreading the area from the SW, will
support widespread showers and possibly a few thunderstorms across
the southern zones as MUCAPE of 250 to 750 J/KG develops across the
the warm sector. While models currently show a lack of sfc based
instability across the area, the impressive deep layer shear
combined with enhanced low-level shear/helicity in the vicinity of
the secondary low pressure development, will need to be closely
watched for damaging wind and isolated tornado potential across the
far southern/southeastern zones overnight.
Overnight lows ranging from lower 40s north to mid 50s south.
&&
.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 350 AM Monday...
Tuesday: A brief lull in the precip is generally expected on Tuesday
as the first in a series of disturbances shifts to the east of the
area. However, skies will continue to remain mostly cloudy to
overcast will low level moisture lingering. Highs on Tuesday are
expected to range from the lower 40s north to the lower to mid 60s
south.
Very tricky forecast for Tuesday night through Wednesday as an upper
trough digs down into the area spurring surface cyclogenesis along
the NC/SC border. While this surface low will quickly move up the
Carolina coast, the upper low will linger behind and bring cold air
down from the north. This in turn with some leftover moisture could
lead to some deformation banding behind the system which could
result in a brief period of rain mixed with snow, or if
precipitation rates are high enough, a quick burst of snow resulting
in some spotty accumulations, particularly across the northern
counties. There are several mitigating factors however, that could
result in very little to no snow. These include warming temperatures
with the heating of the day. Thickness values in the wintry mix to
indeterminate areas of the nomogram for much of the event except the
very tail end where thicknesses drop to levels suggestive of all
snow for a couple of hours when moisture may or may not still be
available. Finally, lift drops off considerably after the surface
low pulls off to the northeast, making it very hard to produce the
rates needed for accumulating snow.
What we are expecting at this time is a period of a few hours of
rain/snow mix during the Wednesday morning hours resulting in very
little to no impacts. There could be some localized heavier snow
showers if precipitation rates are high enough. These should be brief
but could result in some minimal accumulations of less than a half
of an inch if they persist long enough. By the time a changeover to
all snow would occur Wednesday night, there will likely not be
enough lift or moisture left to produce more than a period of
flurries. Bottom line is a late March nuisance event will minimal
impacts.
&&
.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 350 AM Monday...
With the storm system moving up the New England coast and away from
the area, any remaining flurries will come to an end Thursday
morning and clearing will ensue as well as a return to dry weather.
Temperatures will struggle to get out of the 40s across the north
with mid 50s in the south. Dry weather continues on Friday as high
pressure remains in control and a warming trend begins as low to
upper 50s are expected north to south.
The next low pressure system will move out of the Great Plains and
into the Ohio Valley Saturday. Models then split on whether or not
the low takes a southward dive into NC but standing in its way will
be a moderately strong Bermuda high which may help to keep it north
of the area, similar to the EC solution. This will keep us in the
warm sector for the duration of the weekend with highs in the low
50s to low 60s Saturday and upper 50s to upper 60s Sunday.
Precipitation will be determined by the track of the low with the GFS
offering a much wetter solution than the EC. This low and any
associated frontal zone will try to clear the area by Monday morning
but will be quickly followed by several more shortwave disturbances
moving across the deep south and arriving by midweek next week.
&&
.AVIATION /18Z MONDAY THROUGH SUNDAY/...

-- Changed Discussion --

As of 140 PM Monday...
24-Hour TAF Period: Initial VFR conditions this afternoon will
linger through 00Z. Southerly surface flow will strengthen ahead of
a low pressure area moving across the southern tier of the state
tonight. Increased moisture and upglide will induce ceilings to
lower quickly through MVFR to IFR from 00-06Z, with light rain
blossoming and becoming widespread through the period. IFR/LIFR
ceilings are expected after 06Z, to linger past 12Z with marginal
improvement to MVFR towards 15Z.
Looking ahead: Passage of a series of low pressure systems will
result in sub-VFR conditions and periods of rain, probably mixed
with or briefly changing to snow at the northern TAF sites late
Tuesday night into Wednesday. Drying and a return to VFR conditions
from west to east will occur as the fianl trailing low lifts away
from the middle Atlantic coast Wednesday night.